The present invention relates to coating compositions containing a light sensitive diazo substance combined with a colorant, which compositions exhibit improved adhesion to polyester film substrates.
In the art of full color lithographic printing, a plurality of printing plates are provided, each designed to print one of three or four colors such as cyan, magenta, yellow and optionally black. Printing of these colors in superimposed registration on a receptor sheet gives rise to the final full color lithographic image. In order to make the three or four printing plates, the colored original is "separated" photographically, usually by the use of filters, into a set of three or four negatives, each representing one of the colors and containing the amount of that color which must be printed in order for a composite of three or four printed colors to produce the desired total color print.
Prior to the manufacture of each plate for the printing press, the quality of each color separation negative is usually evaluated by the use of proof sheets. These sheets comprise a transparent base sheet coated with a light sensitive composition containing an appropriately colored dye or pigment. In a negative-working proof sheet, the dyed sheet is exposed to actinic light through its respective color separation negative, whereby a photochemical reaction causes the exposed areas to harden in imagewise configuration. Subsequent development and removal of the non-image areas yields a sheet which contains the desired color pattern in the image areas, while being substantially transparent in the non-image areas. In like fashion, similar proof sheets are prepared using the other color separation negatives in each of the colors to be printed. After each of the separate sheets are made, they are assembled together in registry on a white background, whereupon a color proof results. Examples of various kinds of proof sheets and the way they are used may be found in U.S. Pat. Nos. 3,136,637 and 3,671,236; and Canadian Pat. No. 935,018.
As mentioned above, the typical proof sheet adapted for imagewise color exposure consists in the most elementary form of a transparent base sheet coated on one side with a light sensitive composition containing a dyestuff. Most commonly with negative acting materials, the light sensitive composition is composed of a diazo material, used either alone or in combination with a resinous binder, which composition is caused to harden by exposure to a source of actinic light. The hardening occurs as the result of a photopolymerization, condensation, or coupling reaction which renders the light struck areas insoluble in common developer solutions such as aqueous alkaline developers, while the non-light struck areas remain substantially chemically unaltered and soluble. Once developed, the light struck areas of the coating exposed through a negative transparency remain adhered to the base sheet in the form of a positive image.
One of the major problems associated with the preparation and use of the aforementioned proof sheet is poor adhesion between the base sheet and the light sensitive coating. Poor adhesion may give rise to the removal of some of the hardened coating during development, particularly in the margin areas, where the image and non-image portions interface. This is generally caused by a seepage of developer solution into these areas between the hardened coating and the base sheet. The result is that the imaged proof sheet may lack the resolution and image clarity desired, and may even chip off the base as the result of handling.
One technique for improving adhesion involves a physical treatment of the base sheet prior to application of the light sensitive coating, such as a roughening of the surface by sand blasting, by rubbing with abrasives, or by rubbing with metal brushes. This kind of mechanical treatment generally improves adhesion, but seriously reduces the transparency of the base material. Consequently, the finished proof sheets are less than satisfactory from a visual or color standpoint.
Another technique which has been employed commercially is to first coat the untreated base sheet with a thin layer of an adhesive substance which adheres strongly to the base sheet and to the light sensitive coating after it is applied thereto. For example, as disclosed in U.S. Pat. Nos. 2,627,088 and 3,481,736, polyethylene terephthalate film may be coated or "subbed" with a thin adhesive layer composed of a terpolymer of vinylidene chloride/methyl acrylate/itaconic acid, which layer serves to anchor the subsequently applied resinous coating. When these subbed proof sheets are developed, it is found that the coating in the non-image areas of the proof sheet is readily removed and that the resolution and image clarity of the finished sheet are good. However, the adhesive or sub layer tends to become stained by the particular dyestuff present in the light sensitive coating, giving rise to an undesirable tinting of the background areas of the proof sheet which are supposed to be free of residual coating or coloring material. This tinting distorts the true color composite when a plurality of proof sheets are superimposed for proof inspection.
Accordingly, it is an object of the present invention to provide color lithographic proof sheets exhibiting improved adhesion between the transparent base sheet and light sensitive coating composition.
Another object of the invention is to provide color lithographic proof sheets wherein pretreatment of the base sheet, such as by roughening or by the application of an adhesive sublayer, is avoided.
A further object of the invention is to provide color lithographic proof sheets wherein the background or non-image areas of the sheet are not substantially stained.